The file into which some of your preferences are saved across your sessions with Praat. For instance, if you change the font used by the Picture window to Palatino and quit Praat, the Picture-window font will still be Palatino when you start Praat again.

The preferences file is written to disk when you quit Praat, and it is read when you start Praat. It is a simple text file that you can read (but should not edit) with any text editor.